Correct "indexes" to "indices" in Keys doc

Both are valid English, but we use "indices" consistently everywhere
else, and that's generally more common in technical contexts.

(cherry picked from commit e7e4de4)

Author: cuviper

src/map/iter.rs

@@ -626,8 +626,8 @@ impl<K, V> Default for Keys<'_, K, V> {
 /// [values]: RingMap#impl-Index<usize>-for-RingMap<K,+V,+S>
 ///
 /// Since `Keys` is also an iterator, consuming items from the iterator will
-/// offset the effective indexes. Similarly, if `Keys` is obtained from
-/// [`Slice::keys`][super::Slice::keys], indexes will be interpreted relative to the position of
+/// offset the effective indices. Similarly, if `Keys` is obtained from
+/// [`Slice::keys`][super::Slice::keys], indices will be interpreted relative to the position of
 /// that slice.
 ///
 /// # Examples